Field of the Invention
The invention relates to a receiver, more specifically a radio receiver, arranged for receiving FM-signals. Such signals are transmitted by FM-transmitters and are formed by a carrier signal having a given nominal frequency which is changed in response to an information signal. Hereinafter this information signal will be designated a (radio) program.
Description of the Prior Art
FM-signals have the advantage compared with AM-signals that they are less sensitive to interferences which may cause an amplitude change of the carrier signal. As this carrier signal has a very high frequency, the range of an FM-transmitter is considerably shorter than that of an AM-transmitter. For mobile receivers, such as car radios, this is a particularly unpleasant attendant circumstance. If, namely, one moves outside the service area of the FM-transmitter to which the receiver has been tuned and if one wants to continue to follow the radio program transmitted by the transmitter then the receiver must be manually tuned to a different FM-transmitter (other nominal frequency) transmitting the same radio program. In many countries this is possible, as a network of FM-transmitters is present, all FM-transmitters of this network transmitting the same radio program and this radio program being modulated on a carrier signal whose nominal frequency is characteristic of the relevant FM-transmitter.
In order to avoid the necessity of repeated manual tuning of the receiver if one moves outside the service area of one FM-transmitter and arrives in the service area of an other FM-transmitter transmitting the same radio program, it is proposed in reference 1 (see references paragraph below) to store the tuning data from those FM-transmitters which, for example, transmit the same radio program in an addressable memory field of a memory means. This memory field comprises a plurality of addressable memory locations in each of which such a tuning datum has been stored.
In addition, this prior art receiver comprises a control circuit by means of which a tuning datum can be selected from the addressed memory field and applied to a generator. This generator produces a demodulator signal the frequency of which is determined by the tuning datum. This demodulator signal is applied together with the received FM-signal to a mixer circuit, which produces an intermediate-frequency output signal. This output signal is applied to a level detector producing a level signal whose magnitude is proportional to the amplitude of the intermediate-frequency signal. If this level signal becomes smaller than a first threshold value, then the control circuit applies a different tuning datum to the generator. In response to this tuning datum the level detector produces a new level signal. Should this new level signal exceed a second threshold value than this tuning datum is permanently applied to the generator and the receiver remains tuned to the FM-transmitter corresponding to this tuning datum. Should, however, this new level signal be less than the second threshold value then a different tuning datum is again applied to the generator.
Although this prior art receiver is perfectly satisfactory in practice, it nevertheless has the drawback that when the field strength of the FM-transmitter to which the receiver is tuned becomes lower, the quality of the reproduction decreases and that, more specifically, the noise level increases. To prevent this noise level increase from becoming audible, in the prior art receiver an increasing number of high frequency components of the signal to be reproduced are suppressed as the noise level increases. If this noise level exceeds a predetermined threshold value then the receiver may even be switched over (automatically) from stereo to mono reproduction.